Many antenna applications require a conformal (thin) mechanical profile that provides directive beams (high-gain, narrow beamwidth) that can be selectively steered over a pseudo-hemispherical scan volume. Such low-profile two-dimensionally scanned antennas are generically referred to as phased arrays in that the angle between the electromagnetic phase-front and the mechanical normal of the array can be selectively varied in two-dimensions. Conventional phased arrays include a fully-populated lattice of discrete phase-shifters or transmit-receive elements each requiring their own individual phase-control and/or power-control lines.
The recurring costs (component, assembly, and test), prime power, and cooling requirements associated with such electronically controlled phased arrays can be prohibitive in many applications. In addition, such conventional arrays can suffer from degraded ohmic efficiency (peak gain), poor scan efficiency (gain roll-off with scan), limited instantaneous bandwidth (data rates), and data stream discontinuities (signal blanking between commanded scan positions). These cost and performance issues can be particularly pronounced for physically large and/or high-frequency arrays where the overall phase-shift/transmit-receive module count can exceed many thousands of elements.
Variable inclination continuous transverse stub (VICTS) antennas are a different class of antennas that provide the beam steering capabilities of much more expensive electronically scanned phased arrays, but without the need for expensive phase shifters. VICTS antennas are fundamentally traveling wave antennas that mechanically rotate concentric platters (plates) to achieve scanning in the elevation plane.
Since the circular platters rotate about a physical center of the antenna, the aperture extrusions that define the continuous transverse stub (CTS) radiators in the antenna are traditionally designed with identical radiators to enable a symmetric cross section about the rotational center of the antenna. The use of identical radiators helps to reduce production/integration costs and also simplifies the RF modeling analysis.
With the radiator dimensions constrained to be uniformly identical, variable coupling (in order to realize a desired antenna pattern characteristic) is typically achieved via variation of the parallel-plate spacing (the variable air-gap region between the upper and lower platters between which the bounded propagating RF energy travels as it is coupled and subsequently radiated by the stub radiators.) Intentional variation of local parallel-plate spacing immediately below each (fixed geometry) radiating stub allows for customization of the resultant coupling and radiation of RF energy from the common parallel-plate region below. Smaller parallel-spacing (smaller “gaps”) lead to higher coupling whereas larger spacings lead to lower coupling values. It is generally desirable to maximize this dynamic range (ratio of highest coupling to lowest coupling) so as to provide the greatest flexibility in realizing desired antenna pattern characteristics, including beamwidth and sidelobe levels.
Mechanical (and electrical) constraints on the practical range (maximum and minimum) of parallel plate spacing, when paired together with the identical radiator element constraint, limit the achievable coupling range that can be realized in typical array embodiments. This ultimately limits the sidelobe profiles that can be realized thereby limiting the desirable suppression of adjacent satellite interference (ASI) levels and capping the maximum permitted power spectral density (PSD) of a given antenna size when employed in typical satellite communication applications.
VICTS E-Plane taper (sidelobe performance) design is heavily dependent on the available range of coupling one can achieve via variation in the spacing within the parallel plate region. The VICTS antenna designer is limited to a range of coupling values by both mechanical and electrical considerations (constraints).
From an electrical standpoint, setting the parallel plate height too high can introduce unwanted RF moding effects, reducing efficiency, and limiting achievable aperture (sidelobe) tapers as illustrated in FIG. 1. More particularly, FIG. 1 illustrates a CTS radiator 2 having a first (incoming) port P1, a second (outgoing) port P2, a third (coupled) port P3, and separated parallel plates 1 and 3 with associated parallel plate spacing “s”. As can be seen in the graph of FIG. 1, for a given parallel plate spacing “s”, increasing frequency (and correspondingly smaller wavelengths) lead to undesired variability (significant reduction) in the coupling value (|S13|) as the electrical size of “s” approaches a value of one-half-wavelength (λd/2), in this particular example at a frequency of approximately 14.1 GHz. This upper-limit threshold is associated with the presence of undesired multiple modes which propagate between the plates. Shown in the graph of FIG. 1 are |S11| (i.e., the energy reflected back to port 1), |S21| (i.e., the energy transmitted from port 1 to port 2) and |S31| (the energy from port 1 coupled and subsequently radiated through port 3).
From a mechanical standpoint, setting the parallel plate height (spacing) too shallow can lead to undesirable coupling sensitivity to small mechanical variations. Any undesired mechanical tolerance or vibration driven change in parallel-plate spacing, expressed as a percentage of nominal spacing, can become very large (resulting in a correspondingly large undesired variation in coupling) as the nominal parallel-plate spacing varies.
The aforementioned electrical and mechanical factors typically constrain the achievable coupling (maximum versus minimum) via intentional variation in parallel plate spacing to approximately a 6 dB to 7 dB range, thereby restricting the achievable aperture excitation tapers (antenna radiation pattern characteristics) one can design in the plane orthogonal to the radiating stubs (the E-plane.) The H-plane taper is controlled by the feed distribution and is not subject to either of these inhibitors.